[Haskell-cafe] A round of golf

[Don Stewart]

wchogg:
> Hey Haskell,
> So for a fairly inane reason, I ended up taking a couple of minutes
> and writing a program that would spit out, to the console, the number
> of lines in a file.  Off the top of my head, I came up with this which
> worked fine with files that had 100k lines:
> 
> main = do
>  path <- liftM head $ getArgs
>  h <- openFile path ReadMode
>  n <- execStateT (countLines h) 0
>  print n
> 
> untilM :: Monad m => (a -> m Bool) -> (a -> m ()) -> a -> m ()
> untilM cond action val = do
>  truthy <- cond val
>  if truthy then return () else action val >> (untilM cond action val)
> 
> countLines :: Handle -> StateT Int IO ()
> countLines = untilM (\h -> lift $ hIsEOF h) (\h -> do
>                                                 lift $ hGetLine h
>                                                 modify (+1))
> 
> If this makes anyone cringe or cry "you're doing it wrong", I'd
> actually like to hear it.  I never really share my projects, so I
> don't know how idiosyncratic my style is.

This makes me cry.

    import System.Environment
    import qualified Data.ByteString.Lazy.Char8 as B

    main = do
        [f] <- getArgs
        s   <- B.readFile f
        print (B.count '\n' s)

Compile it.

    $ ghc -O2 --make A.hs

    $ time ./A /usr/share/dict/words
    52848
    ./A /usr/share/dict/words 0.00s user 0.00s system 93% cpu 0.007 total

Against standard tools:

    $ time wc -l /usr/share/dict/words
    52848 /usr/share/dict/words
    wc -l /usr/share/dict/words 0.01s user 0.00s system 88% cpu 0.008 total

-- Don